Phototroph-specific nitrogen isotopes detail N2-fixation increases at the end-Triassic extinction

The end-Triassic mass extinction (~201.6 Ma) marked fundamental ecological and environmental disruptions triggered by the emplacement of the Central Atlantic Magmatic Province. One critically overlooked aspect of this event is the nitrogen cycle, despite its central role in regulating the carbon cycle and supporting marine ecosystems. To investigate these changes, we analyzed compound-specific nitrogen and carbon isotopes of sedimentary metalloporphyrins derived from phototrophs at a key coastal ETE locality: St. Audrie’s Bay, UK. Our porphyrin analysis provides robust constraints on the isotopic compositions of major groups of phototrophs throughout the photic zone, specifically purple and green sulfur bacteria inhabiting in and below the chemocline, along with algae and/or cyanobacteria in the overlying oxygenated surface waters. These primary producer-specific nitrogen isotopes indicate a substantial reorganization of the coastal nitrogen cycle following the end-Triassic mass extinction. In the immediate extinction aftermath, N2-fixation contributed up to ~90% of primary production and averaged between ~50% and ~65%. These values are substantially higher than previously reported for anoxic coastal environments, which best represent the depositional conditions at St. Audrie’s Bay. This pronounced shift toward N2-fixation suggests strongly stratified marine systems subjected to rapid CO2-forcing undergo fundamental nitrogen cycling restructuring, with important implications for future climate scenarios. “Primary producer-specific nitrogen isotopes indicate a substantial reorganization of the coastal nitrogen cycle following the end-Triassic mass extinction; analyzed using compound-specific nitrogen and carbon isotopes of sedimentary metalloporphyrins derived from phototrophs at St.